Duplication usually happens after or during syncing. I have the same problem in the past. But I guess you could try using a duplicate remover like Scrubly Duplicate Contacts Remover. This really works well for me. They have other features you can use. Just give it a try.:)
I Hope this helps...
Duplication takes place after or during syncing, it's really frustrating and can cause other conflicts. I have the same problem before so I know how it feels. I suggest you can do the same as I do. You may use duplicate remover like Scrubly Duplicate Contacts Remover, this really works well for...